This invention relates to the production of polyurethane urea elastomers having improved mechanical properties by the casting process. The improvement is obtained by using certain isomer mixtures of diphenylmethane diisocyanate as isocyanate component in the synthesis of the elastomers.
It is known, for example, from Vieweg-Hochtlen, Kunststoff-Handbuch, Vol. VII, Carl-Hanser-Verlag, Munich, 1966, pages 206-297, that NCO-group-containing prepolymers based on relatively high molecular weight polyols (for example, polyethers or polyesters) and symmetrical diisocyanates (such as, for example, 4,4'-diphenylmethane diisocyanate or 1,5-naphthylene diisocyanate) can be processed by casting with short-chain diols (preferably 1,4-butane diol) as chain-extending agent to form high-quality elastomers. As is known, the elastomers can only be obtained in a satisfactory quality by using chemically pure diisocyanates as the isocyanate component. Thus, as little as 10% of 2,4'-diphenylmethane diisocyanate, for example, in 4,4'-diphenylmethane diisocyanate, is sufficient to produce a distinct reduction in the quality of the elastomers. Shoe soles produced therefrom become brittle relatively quickly, by comparison with those based on pure 4,4'-diphenylmethane diisocyanate.
It is also known that polyurethane urea elastomers can be produced by the casting process. In this case, prepolymers containing NCO-groups generally synthesized with tolylene diisocyanate, are reacted with aromatic diamines as chain-extending agent. High-quality elastomers are also obtained in this way, as described in U.S. Pat. No. 3,188,302 using 3,3'-dichlorodiphenylmethane-4,4'-diamine as the chain-extending agent or in German Auslegeschrift No. 1,803,635 using 3,5-diamino-4-chlorobenzoic acid isobutyl ester.
If an attempt is made to produce polyurethane urea elastomers such as these by the casting process using diphenylmethane diisocyanate as the isocyanate component, problems arise both in regard to the processibility of the prepolymer and in regard to the mechanical properties of the end product.
Prepolymers synthesized from 1 mole of a relatively high molecular weight polyhydroxyl compound and 2 moles (or less) of 4,4'-diisocyanatodiphenylmethane have a very high viscosity, which gives rise to serious processing problems, particularly in high-pressure machines. Prepolymers having viscosities of more than 2000 mPa.s can no longer be satisfactorily mixed in high-pressure machines. On the other hand, the temperature cannot be increased significantly beyond 70.degree. C. in order to reduce the viscosity because the reaction velocity increases and the stability of the prepolymers distinctly decreases with increasing temperature.
Because of the high viscosity of the prepolymers based on 4,4'-diphenylmethane diisocyanate, the casting mixtures show poor flow properties and are difficult to homogenize with the result that it is not possible to cast moldings of large volume. Another disadvantage of prepolymers such as these lies in their unsatisfactory storage stability due in particular to dimerization of the isocyanate groups into uretdione groups.
One way of reducing the viscosity of the prepolymers while, at the same time, improving their storage stability is to use more than 2 moles of isocyanate per mole of the relatively high molecular weight diol during their production, so that the prepolymer contains relatively large quantities of free monomeric 4,4'-diisocyanatodiphenylmethane. However, this approach is attended by the disadvantage that, even in cases where diamines of low reactivity are used, the reaction velocity during the chain-extending reaction increases greatly with increasing free diisocyanate content of the prepolymer. This results in casting times of the reaction mixture which are too short.
If an attempt is made to reduce the high reaction velocity of the casting mixture by diluting the amine component with polyols, a further problem arises, i.e, the highly symmetrical rigid segments which are formed from 4,4'-diphenylmethane diisocyanate and aromatic diamines are not sufficiently soluble in the reaction mixture. Accordingly, most of these rigid segments are precipitated without being incorporated in a polyurethane molecule. As a result, the mechanical properties of these elastomers are unsatisfactory.
Finally, prepolymers based on pure 4,4'-diphenylmethane diisocyanate cannot be processed satisfactorily with highly reactive aromatic diamines, such as, for example, alkyl-substituted tolylene diamines and phenylene diamines, because of the high reactivity of the two isocyanate groups.
Accordingly, the object of the present invention is to provide casting systems which, on the one hand, result in elastomers having the outstanding property spectrum of products based on 4,4'-diisocyanatodiphenylmethane, but which on the other hand have improved processibility, even where the highly reactive alkyl-substituted phenylene diamines and diaminodiphenylmethanes are used.
It has surprisingly been found that this object may be achieved by using certain isomer mixtures of diisocyanatodiphenylmethane during the synthesis of the prepolymer component. This must be regarded as surprising because, from the above-discussed experiences with diol-extended casting systems, it had been expected that the properties of polyurethane urea elastomers would also be considerably impaired by using isomer mixtures as opposed to pure 4,4'-diisocyanatodiphenylmethane.